Mechanisms for the synthesis of conjugated enynes from diphenylacetylene and trimethylsilylacetylene catalyzed by a nickel(0) complex: DFT study of ligand-controlled selectivity.

Density functional theory (DFT) was utilized to elucidate the reaction mechanisms of and the key factors that influence the Ni(0)-catalyzed cross-dimerization and -trimerization of trimethylsilylacetylene (R1) and diphenylacetylene (R2). Calculated results revealed that the electron-donating ability of the ligand plays a crucial role in determining the regionselectivity of this tandem reaction. The use of strongly electron-donating ligands favors the formation of cross-dimer intermediates, whereas cross-trimer products can easily be synthesized using weakly electron-donating ligands. A simple method of estimating the electron-donating abilities of different ligands based on the Mulliken charge distribution of the ligand-ligand pair was employed. The present theoretical results allow us to elucidate the reaction mechanisms for and to identify the factors that exert the greatest influence on the ligand-controlled cross-dimerization and -trimerization of trimethylsilylacetylene and diphenylacetylene. Guidelines for the design of novel ligand systems with Ni(0) catalysts are also proposed.